Conditioning of the masseter inhibitory reflex by homotopically applied painful heat in humans1

During contraction of the jaw-closing muscles, afferent input from the intraoral and perioral region can elicit two bilateral suppression periods (SP1 and SP2, respectively) in the masseter electromyogram (EMG). Non-painful electrical stimulation 2 cm from the left labial commissure was used in the present study to evoke these trigeminal inhibitory reflexes. The subjects maintained a level of 50% of their maximum masseter EMG. The degree of suppression was quantified as the percentage suppression of the mean EMG activity in a fixed post-stimulus interval (SP2, 40–90 ms). Further, brief (200 ms) painful radiant heat conditioning stimuli were delivered to the ipsilateral cheek, in order to investigate the influence of nociceptive input on the (non-nociceptive) trigeminal masseter inhibitory reflex. Nine different conditions combining radiant heat and electrical stimuli were used. Twelve stimuli were presented for each condition. The radiant heat preceded the electrical test stimuli by fixed inter-stimulus intervals (ISI), ranging from 100 ms to 500 ms. At 250–350 ms ISIs, the bilateral SP2 suppression was significantly reduced to less than 10%, in comparison to an average suppression degree of 32.5% without conditioning stimuli. The subjects perceived the heat stimulus before the electrical stimulus for a majority of the 12 pairs of stimuli at these ISIs. No differences were found in the VAS ratings for the different conditions. For the contralateral SP1, larger suppression was seen for the 300 ms ISI compared with stimulation without conditioning heat stimuli. Onset and offset for the SP1 was, however, only detected in three subjects using a criteria of 20% suppression of the pre-stimulus activity. A pre-pulse inhibitory effect onto inter-neurons in the SP2 pathways or habituation of the same inter-neurons by the heat stimuli are suggested as possible explanations for the interaction between the non-nociceptive and nociceptive input in the present study.